The present invention relates to an information recording medium for repeatedly recording/erasing information using a phase change recording layer in which the atomic alignment can be changed by irradiation of an electron beam or the like to change optical characteristics, and for recording/reproducing information by detecting changes in optical characteristics, and an information recording/reproduction method using this recording medium.
A recording erasable information recording medium has the following structure. FIG. 1 is a schematic sectional view showing the structure of an example of a conventional information recording medium. This information recording medium is constructed by a substrate 101, a first protective layer 102, a recording layer 103, a second protective layer 104, and a reflective layer 105 shown in FIG. 1. The substrate 101 is made of a glass or a plastic material (e.g., a polymethyl methacrylate resin or a polycarbonate resin), and the first and second protective layers 102 and 104 are made of ZnS, SiO.sub.2, Al.sub.2 O.sub.3, or a mixture of them. The recording layer 103 is made of a chalcogenide material such as GeSbTe. These layers can be formed by deposition methods such as vacuum deposition or sputtering. The reflective layer 105 is made of Al, Au, or an alloy containing Ti, Mo, Zr, or Cr and Al or Au as a base material.
Information can be recorded/erased as follows using this information recording medium. The entire surface of the information recording medium is irradiated and heated with a light beam to set the recording layer in a high crystallinity state (to be referred to as a crystalline state hereinafter), i.e., a state wherein atoms are relatively regularly aligned. To write or record information, the recording medium is irradiated with a short, strong pulse beam to heat, fuse, and quench the recording layer. Then, the pulse beam irradiated portion changes to a low crystallinity state (to be referred to as an amorphous state hereinafter), i.e., a state in which the atomic alignment is disturbed.
Since the crystalline state and the amorphous state are different in atomic alignment, optical characteristics, e.g., the transmittance and the reflectivity change to allow recording of information. Information written in this way can be erased by irradiating the recorded portion with a long, weak pulse beam, and heating and slowly cooling it at the melting point or less. This is because the recorded portion returns to the original crystalline state.
FIG. 2 is a view for explaining the laser power in recording during erase. As shown in FIG. 2, the above state can be realized by overwriting a new recorded portion using a laser power (recording power) prepared by superposing strong, short pulses on a weak continuous laser power (bias power), while erasing (crystallizing) a recorded portion (amorphous state) formed previously.
Since a practical recording medium uses a reflectivity change between the crystalline and amorphous states as a signal, the thicknesses of the respective layers are designed in consideration of optical interference at the interface between the protective layer and the recording layer and the interface between the protective layer and the reflective layer. These thicknesses are desirably optimized to attain a large optical reflectivity change in accordance with the optical constants of materials used in the medium.
FIG. 3 shows the calculation results for the amounts of change in the reflectivities and reflectivity of the recording (amorphous) state and the non-recording (crystalline) state with respect to the thickness of the second protective layer when, e.g., the recording layer is made of GeSbTe, the reflective layer is made of Al, and the protective layer is made of ZnS:SiO.sub.2. In FIG. 3, graph 111 represents the reflectivity of a recorded portion; graph 112, the reflectivity of a non-recorded portion; and graph 113, the reflectivity change amount.
From these results, the thickness capable of attaining the maximum reflectivity change amount is about 100 .ANG. to 200 .ANG. or 1,400 .ANG. to 1,500 .ANG.. For the thickness of 100 .ANG. to 200 .ANG., information is recorded in the direction in which the reflectivity decreases. For the thickness of 1,400 .ANG. to 1,500 .ANG., information is recorded in the direction in which the reflectivity increases.
The two thicknesses can optically attain a large reflectivity change amount. However, an increase in thickness of the protective layer obstructs the flow of heat from the recording layer to the reflective layer, and modulation of the laser power cannot satisfactorily control quenching, degrading the recording characteristics (e.g., T. OHTA et al., JJAP VOL. 128 (1989), SUPPLEMENT 28-3, pp. 123-128). When a recording material such as GeSbTe is used for the recording layer, the thickness of the second protective layer is conventionally set at 100 .ANG. to 200 .ANG., and information is recorded in the direction in which the reflectivity decreases.
Digital data recording methods using this optical disk include mark position recording and mark length recording. FIG. 4 is a schematic view showing an example of mark position recording, and FIG. 5 is a schematic view showing an example of mark length recording. As shown in FIG. 4, in mark position recording, recording marks 6 having the same shape are formed, and information can be obtained from the interval between the centers of the marks. As shown in FIG. 5, in mark length recording, information is recorded while changing the lengths of recording marks 7, and information can be obtained from the recorded mark length.
Conventionally, information is recorded on only either the grooves or lands formed in an optical disk. Recently, land/groove recording of recording information on both the lands and grooves is also proposed. FIGS. 6 and 7 are a perspective view and a plan view, respectively, showing an example of the state wherein information is recorded on both the lands and grooves. As shown in FIGS. 6 and 7, in this recording, recording marks 143 are formed on both lands 141 and grooves 142 by irradiating the optical disk with a recording beam 144.
As land/groove recording, e.g., N. Miyagawa et al., J.J. Appl. Phys. Vol. 32 (1993), pp. 5,324-5,328 discloses a method capable of reducing crosstalk from an adjacent track by forming a groove with a proper depth (about .lambda./6) in a phase change optical disk.
For a thickness suitable for conventional mark position recording, the recording area (amorphous region) on the disk is relatively smaller than the remaining non-recording (crystalline) region. In mark length recording, however, the recording area on the disk is larger than in mark position recording. For example, assuming the diameter of the recording mark to be 0.78 .mu.m, the area ratio of the recording region to the non-recording region on the disk is 26% for mark position recording and 44% for mark length recording. If these recording methods are adopted for a conventional recording medium on which information is recorded in the direction in which the reflectivity decreases, the average reflectivity becomes lower upon reproduction than before recording as the area ratio of the recording region becomes higher.
In mark position recording, even if information is recorded in the direction in which the reflectivity decreases, a certain degree of reflected light quantity can be obtained, and the signal can be reproduced by focusing and tracking with a general optical disk drive. In mark position recording, however, when the interval between recording marks is decreased for a higher density, and information is recorded or reproduced, the area of the recording region with respect to the total area increases on the disk in which the reflectivity decreases upon recording. As a result, the average reflectivity becomes 60% or less the previous reflectivity. For example, using thickness A in FIG. 3 leads to an average reflectivity of 10% or less. This thickness cannot attain a reflected light quantity necessary for focusing and tracking because stable focusing and tracking generally require a reflectivity of at least 10%. To increase the reflectivity, the thickness of the protective layer must be increased, which obstructs maximization of the reflectivity change amount.
In the conventional structure in which the reflectivity decreases upon recording, if light is prevented from escaping from the lower surface of the reflective layer, the absorptivity is large at a recorded portion and small at a non-recorded portion. Since the absorptivity is different between a new mark overwritten on a recorded portion and a new mark overwritten on a non-recorded portion, the temperature rise upon recording becomes different between these portions. Further, since the crystalline non-recorded portion must be fused with latent heat, the temperature rise upon heating with the same laser power becomes more different. The size of the recording mark formed by fusion and quenching varies. For this reason, in the recording scheme of holding information at the edge of the recording mark, the edge position is fluctuated depending on the location.
To solve this problem, the following method is proposed. In the conventional layer structure (substrate/first protective layer/recording layer/second protective layer/reflective layer), a metal layer (Japanese Patent Application No. 5-221905) or a high-refractive-index layer (Japanese Patent Application No. 6-52494) is inserted between the substrate and the first protective layer. By using interference at the interface between the layers and interference from the substrate/metal layer or the high-refractive-index layer, a thickness structure of layers in which the reflectivity increases upon recording is employed instead of the conventional thickness structure of layers in which the reflectivity decreases upon recording.
In the disk having this structure, when data is recorded on the lands and grooves, the signal amplitudes become different between the lands and grooves due to the phase difference between the crystalline and amorphous recording materials, and the phase difference caused by the depth difference between the lands and grooves.
Moreover, recording information on the lands and grooves thermally influences an adjacent track. In recording, information of the adjacent track is erased.